Photovoltaic devices utilize specific conductivity characteristics of materials generally referred to as semiconductors, whereby solar energy or radiation is converted to useful electrical energy. This conversion results from the absorption of photon energy in the active region of the cell, whereby some of the absorbed energy causes the generation of electron-hole pairs. The energy required for the generation of electron-hole pairs in a semiconductor material is referred to as the band gap energy and generally is the minimum energy required to excite an electron from the valence band to the conduction band.
There are two principal measures of the utility of photovoltaic devices. First is the efficiency of the device, which is an ascertainable percentage of the total photon energy converted to useful electrical energy. High efficiency photovoltaic devices made of crystalline materials maximize efficiency by minimizing internal lattice defects which can cause internal shorts and boundary defects, thereby reducing the conductivity and recombination characteristics of the devices. The second measure of the utility of a photovoltaic device is its cost. Single crystal devices are complex and costly to produce, and do not readily lend themselves to high volume production.
One approach to reducing the cost of photovoltaic devices is to utilize polycrystalline materials and a heterojunction. A heterojunction refers to the active junction formed at the interface between two or more dissimilar materials, such as cadmium sulfide and cadmium telluride, as taught by Basol, et al. in U.S. Pat. No. 4,388,483. Basol, et al. describe thin-film photovoltaic cells wherein the active layer comprises at least one of the metal elements of Group IIB of the Periodic Table of Elements and one of the non-metal elements of Group VIA of the Periodic Table of Elements. One feature of such photovoltaic devices is the use of extremely thin active layers. As an example, Basol, et al. utilized a cadmium sulfide layer on the order of 0.02-0.05 micrometer thick and a cadmium telluride layer on the order of about 1.3 micrometers thick. While such economy of material has obvious advantages, it has also presented an unexpected concern with respect to current collection.
Such thin-film photovoltaic devices typically comprise an optically transparent substrate, a first semiconductor layer formed on the transparent substrate, a second semiconductor layer of opposite conductivity type from the first semiconductor layer and forming a junction with the first layer, and a back contact comprised of a conductive film. When the substrate is not electrically conductive, a transparent electrically conductive front contact is disposed between the substrate and the first semiconductor layer; the front contact generally being a layer of a transparent conductive oxide. This layer functions as a current collector for the photovoltaic device. Transparent conductive oxides, such as indium tin oxide, indium oxide, zinc oxide, and tin oxide however, are not efficient current collectors in cells of any appreciable size, that is, greater than about one square centimeter, due to their inherent resistivity which is on the order of about 10 ohms per square at thicknesses necessary for good optical transmission. The transparent conductive layer must be supplemented with more efficient current collection means such as a front contact current collector grid. Formation of a front contact current collector grid for thin-film photovoltaic devices presents novel concerns as the general thickness of a front contact grid-type current collector disposed in contact with the transparent conductive layer typically exceeds the thickness of the first active semiconductor layer and extends into the second active semiconductor layer. This geometry creates problems with respect to shorting of the device and to uniform formation of the semiconductor layers themselves.
Generally, a front contact current collector grid is deposited onto a transparent conductive layer and followed by subsequent depositions of the active semiconductor layers. This procedure has several drawbacks when applied to the formation of a front contact current collector grid for a photovoltaic device when the thickness of the grid exceeds the thickness of the first semiconductor layer.
A first concern relates to diffusion of the grid material into the semiconductor layers. If subsequent process steps for the formation of the semiconductor layers include treatments at elevated temperatures that encourage diffusion, or if the semiconductor layers are electrodeposited in a bath that may dissolve the grid material, then the grid and the semiconductor layers may be adversely affected. In the latter instance, the grid may be deteriorated and the bath may become contaminated with leached grid material, which then appears in the deposited semiconductor layer.
A second concern arises with regard to the deposition of semiconductor layers subsequent to the formation of the current collector grid. The deposition may not be uniform or complete due to the presence of the current collector grid, which has a relatively large thickness. It has been seen that the electrodeposition of semiconductor materials near such a grid is imperfect, often leaving exposed portions of the transparent conductive layer which may become eroded. Under such circumstances, the grid is insulated from the semiconductor layers and cannot be an effective current collector. This has the adverse result of increasing the series resistance of the photovoltaic device, and may ultimately cause failure of such a device.
What is needed in the area of efficient current collection in photovoltaic devices is a stable front contact current collection grid not suffering from the above-described deficiencies.
It is therefore one object of the present invention to provide a front contact current collector grid for photovoltaic devices, which current collector grid is not accompanied by the above-identified shortcomings.
It is another object of the present invention to provide a photovoltaic device having a stable, front contact current collector grid incorporated therein.
It is yet another object of the present invention to provide a method for the formation of a photovoltaic device that has a stable front contact current collector grid incorporated therein.
These and additional objects of the present invention will become apparent to one skilled in the art from the below description of the invention and the appended claims.